In the related art, for example, there has been known an apparatus in which other vehicle is detected by a millimeter wave radar device, the vehicle density between the other vehicle present within a predetermined distance from a host vehicle and the host vehicle is calculated and, further, whether the traveling state of the host vehicle is a cause of traffic congestion is determined by using critical density in accordance with the speed of the host vehicle to notify a driver of the determination result (for example, refer to patent document 1).